added image button role on iOS

Summary:
Because we're now separating accessibilityTraits into accessibilityRole and accessibilityState, we're going to only allow one role to be set, and allow on preset combinations of roles that make sense.

This change adds iOS functionality to the role image button because a popular accessibilityTrait combination is image and button.

Reviewed By: ikenwoo

Differential Revision: D8847012

fbshipit-source-id: da386dbf82cb3854d14c228a1116da9f4067fe93

React/Views/RCTViewManager.m

@@ -33,6 +33,7 @@ @implementation RCTConvert(UIAccessibilityTraits)
   @"header": @(UIAccessibilityTraitHeader),
   @"search": @(UIAccessibilityTraitSearchField),
   @"image": @(UIAccessibilityTraitImage),
+  @"imagebutton": @(UIAccessibilityTraitImage | UIAccessibilityTraitButton),
   @"selected": @(UIAccessibilityTraitSelected),
   @"plays": @(UIAccessibilityTraitPlaysSound),
   @"key": @(UIAccessibilityTraitKeyboardKey),